Abstract

The phase transition from tetragonal to hexagonal of crystalline powders prepared by solid state reaction has been studied by x-ray diffraction and x-rayabsorption fine structure (XAFS). At the low Co doping level of , the structure of the samples is tetragonal, and it is transformed gradually to a hexagonal structure upon increasing Co content to . The detailed analysis of -edge XAFS indicated unambiguously that the doped Co ions are substantially incorporated into the host and likely serve as a trigger in leading the structure transition. On the basis of the formation energy calculations, we find that there exists a critical Co concentration in the range of 0.027–0.055 for inducing the tetragonal-to-hexagonal structure transition of , in good agreement with the experimental observations.